Radar systems and devices are well known. A typical radar system transmits an electromagnetic signal and receives reflections of the transmitted signal. The time delay between the transmitted and received signals is indicative of the distance of objects causing the reflections.
In some radar systems, a phase shift (also referred to as phase rotation) is used. A phase shift of 90° may be used to better detect reflected signals in noise (quadrature detection), as disclosed in U.S. Pat. No. 3,942,178, for example. In U.S. Pat. No. 3,942,178, the phase-shifted transmitter signal is fed directly to the receiver but the transmitted signal is not phase-shifted.
A phase shift of 180° may be used to better distinguish reflected signals from transmitted signals. In radar systems using such a phase shift, the transmitted signal may be periodically phase shifted. The period is typically chosen such that the phase shift occurs between two time frames of the signal, thus altering the phase every time frame, where a time frame may, for example, correspond with a “chirp” in an FMCW (frequency modulation continuous wave) signal.
In radar receivers, high pass filters with low cut-off frequencies are often used to attenuate low frequency interferences, for example in automotive applications (the “bumper effect” caused by reflections from car bumpers). In addition, low pass filters may be used to limit the effective signal bandwidth to be converted by any analog-digital converters.
When shifting the phase of the transmitted signal, however, filters processing the received signal may go into saturation. This is not a problem if this occurs between time frames and if the filters are no longer in saturation when the next time frame starts. However, when a very short inter-frame period is used, the filters may still be in saturation when the next time frame starts, which will impede the proper functioning of the radar device.